The present invention relates to storage facilities for spent nuclear fuel assemblies, and more particularly to a storage rack which maximizes the number of fuel assemblies that can be stored in a given space.
Nuclear reactors consist of an array of fuel rods containing the nuclear fuel. These fuel rods are metal tubes which may be from 8 to 15 feet in length and about 1/2 inch in diameter, and they are supported in groups in fuel assemblies which may typically consist of from 49 to as many as 300 individual fuel rods. The large reactors utilized for power generation contain a large number of these fuel assemblies arranged in a suitable configuration.
After an extended period of operation, which may be a year or more for example, the irradiated or spent fuel assemblies must be removed from the reactor and replaced. The spent fuel rods are then chemically reprocessed and renewed. The available chemical reprocessing facilities, however, are limited in capacity and the spent fuel assemblies must be stored at the nuclear reactor site until they can be shipped to a reprocessing facility. Storage of nuclear fuel assemblies presents serious problems since they are dangerously radioactive and must be kept submerged in water in a storage pit. Furthermore, since neutrons are still being emitted from the spent fuel assemblies, they must be stored in such a manner that criticality of the collection of fuel assemblies is prevented. This has heretofore required a very substantial spacing between fuel assemblies in the storage pit, so that they are separated by a considerable thickness of water which functions as a moderator and prevents criticality. In some cases, curtains of a poison, or neutron-absorbing material, have also been used between fuel assemblies but since such material does not capture fast neutrons, the necessity for a large spacing was not avoided. The result of this is that a very large space has been required for the storage of a substantial number of spent fuel assemblies, and many nuclear reactor plants do not have adequate storage facilities for the number of fuel assemblies which must be stored.